Although it will become evident to those skilled in the art that the present invention is applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the invention and its background will be described principally in the context of a specific example of such devices, namely, cardiac pacemakers for providing precisely controlled stimulation pulses to the heart. The appended claims are not intended to be limited, however, to any specific example or embodiment described herein.
Pacemaker leads form the electrical connection between the cardiac pacemaker pulse generator and the heart tissue which is to be stimulated. As is well known, the leads connecting such pacemakers with the heart may be used for pacing or for sensing electrical signals produced by the heart or for both pacing and sensing in which case a single lead serves as a bidirectional pulse transmission link between the pacemaker and the heart. An endocardial type lead, that is, a lead which is inserted into a vein and guided therethrough into a cavity of the heart, includes at its distal end an electrode designed to contact the endocardium, the tissue lining the inside of the heart. The lead further includes a proximal end having a connector pin adapted to be received by a mating socket in the pacemaker. A flexible, coiled conductor surrounded by an insulating tube or sheath of biocompatible material, couples the connector pin at the proximal end with the electrode at the distal end.
The electrode tip of an endocardial lead must be anchored to the heart to prevent it from becoming dislodged or dislocated. A number of methods, both passive and active, have been devised for this purpose. In accordance with one known passive fixation technique, a plurality of flexible tines are molded integrally with the insulative sheath covering the coiled electrical conductors and extend rearwardly at an acute angle relative to the longitudinal axis of the lead. Following implantation of the lead, the tines become entangled in the trabecular network thereby securing the electrode position. Since the tines can flatten against the lead body and thus reduce its diameter, tined leads are often suitable for introduction through small blood veins. Other known passive fixation techniques include collar electrodes which have one or more conical projections of silicon rubber or other biocompatible flexible material behind the electrode tip. Like the tines, the cone becomes entangled in the trabecular network inside the heart, thereby anchoring the electrode tip. In yet another known approach which is advantageous if relocation of the electrode tip becomes necessary, projecting, flexible fins are used to provide stable anchoring.
Irrespective of the passive fixation technique employed, the anchoring section of an endocardial lead is the portion of the tip that has the largest cross sectional area. It is desirable to minimize this area to facilitate passage of the lead through small diameter blood veins by minimizing the resistance to insertion and removal of the lead. It is also desirable to minimize the cross sectional area of the tip portion of the lead so as to reduce the diameter of the introducer sleeve where implantation is effected by means of a lead introducer. Since the purpose of the introducer is to provide direct entry of the endocardial lead into a vessel, it is important to minimize the size of the opening in the vessel so as to minimize trauma at the introduction site. In the case of tined leads, one expedient for minimizing lead cross sectional area is to provide a recessed portion of the insulating sheath just behind the tines. During implantation the recessed portion receives the folded tines which lie flat within that portion and substantial flush with the outer surface of the insulative sheath.
As is known, many of today's intravascular endocardial leads are multipolar in which, besides an electrode at the tip, one or more ring electrodes are incorporated in the distal end portion of the lead for transmitting electrical stimulation pulses from the pulse generator to the heart and/or to transmit naturally occurring sensed electrical signals from the heart to the pulse generator. Thus, by way of example, in a typical bipolar lead having a tip electrode and a ring electrode, two concentric conductor coils with insulation in between are carried within the insulative sheath. One of the conductor coils connects the pulse generator with the tip electrode while the other conductor coil, somewhat shorter than the first conductor coil, connects the pulse generator with the ring electrode positioned behind the tip electrode. More recently, to reduce the outside diameter of multipolar leads, the individual conductors are insulated and instead of being concentric all of the conductor coils are wound on the same diameter. Thus, in a multipolar lead employing this technique, the various coil conductors are interleaved along the same coil diameter thereby helping to reduce the overall diameter of the lead.
Despite the foregoing and other techniques for reducing the cross sectional area of the tip portion of endocardial leads, still further size reduction is desirable.
Accordingly, an overall object of the present invention is to further reduce the cross sectional area of the tip portion of an endocardial lead employing passive fixation means.